This invention generally relates to filtering noise from a data signal and, more particularly, to filtering data signals received by receiving devices with optocoupler based filtering circuits.
Single-ended data transmission lines are inexpensive unidirectional transmission lines for transmitting data signals from a network to a receiving device (receiver). Because of loading considerations, receivers utilizing single-ended lines require a high input impedence interface circuit to electrically connect the line to the receiver. Since the input impedence of the interface circuit is high, a relatively small change in the current through the line can significantly increase the voltage across the interface, thus corrupting the transmitted data signal. Accordingly, receivers utilizing single-ended transmission lines are very susceptible to noise in the transmission line. Such noise, which may be caused by surrounding energy fields, typically propagates at frequencies that are greater than about one megahertz.
The prior art has addressed this problem by including internal filtering devices within the receiver for filtering high frequency noise (frequencies greater than about one megahertz) from single-ended data transmission signals. Among those filters are passive filtering devices, which typically include a combination of resistors, capacitors, and/or inductors. Passive filtering devices are inefficient for such purposes, however, because they require that one or more of the resistors in the filter have a relatively high resistance value. This large resistance limits current flow through the line, thus impeding data transmission to the receiver.
Non-isolating active filtering devices also have been suggested for filtering high frequency noise from data transmission signals. Such filters often are expensive, however and can be damaged when a high energy noise burst is coupled into the transmission line. When the filtering device is damaged, the noise can couple with the receiver, thereby damaging the receiver and/or corrupting the received data transmission signal.
In response to these problems, electrically isolating optocouplers have been used for electrically isolating receivers from noise bursts through transmission lines. This isolation commonly is referred to as xe2x80x9cgalvanic isolation.xe2x80x9d In simplified terms, an optocoupler has a light emitting device (e.g., a light emitting diode, a/k/a xe2x80x9cLEDxe2x80x9d) that is driven by a data current signal, and a photodetector for detecting the light emitted by the light emitting device. The photodetector may be a phototransistor, which requires a biasing source for powering the phototransistor. The intensity and frequency of the light emitted by the LED vary in proportion to the intensity and frequency of the data current signal. When the photodetector detects the emitted light, it first regenerates an attenuated version of the original electrical signal from the received light, and then transmits the regenerated signal to the receiving device.
The degree of attenuation of the signal through an optocoupler is ascertained by the current transfer ratio (CTR), which is the ratio of current transmitted by the optocoupler to the current received by the optocoupler. Since the CTR of an optocoupler typically is less than one, the biasing source may be configured to restore the amplitude of the regenerated signal back to that of the received signal. Like the optocoupler, the biasing source is internal to the receiver.
Problems arise when an existing receiver does not include a low pass filter for minimizing the effects of high frequency noise from a received data signal. Such receivers consequently must be internally retrofitted to include such a filter. This requires that the receiver be disassembled and/or redesigned for the filter and biasing source to be appropriately connected. This redesign process is time consuming and inefficient. Moreover, many receivers cannot effectively be retrofitted with such filters even with a significant device redesign. Those receivers therefore continue to be susceptible to the adverse effects of high frequency noise.
Accordingly, it would be desirable to have a device that limits the adverse effects of high frequency line noise for receivers that do not include electrically isolated, internal noise reduction filters. It also would be desirable for such device to be used with such a receiver without requiring that the receiver be internally retrofitted to accommodate such device.
In accordance with the principles of the invention, an isolating data signal attenuator for filtering high frequency noise from a data signal is detachably connectible to the exterior of a receiver. The attenuator includes an optocoupler, a biasing source for powering the optocoupler and restoring the amplitude of the originally transmitted signal, and a connector for connecting the attenuator to the exterior of a receiving device. When connected to the receiver, the attenuator electrically connects a single-ended data transmission line to the receiver. The external connection of the attenuator to the receiver thus enables existing receivers to have the benefits of an optocoupler based attenuator without the necessity of an internal retrofit. Accordingly, the receiver is both electrically isolated from the network and substantially free of undesirable high frequency noise transmitted through the transmission line.